GPS doesn't show true data when moving in Gazebo

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Hello everyone!

I have installed Gazebo 8 with ROS Kinetic on Ubuntu 16.04 LTS. I have built a custom robot model in Gazebo and added a libhector_gazebo_ros_gps plugin. Also attached a base_footprint on the robot. But when i run rostopic echo /fix and rostopic echo /fix_velocity, none of them shows true data.

Do i need to build a map first? I tried to build a map through turtlebot tutorials, but i see no map on Rviz either. I have been searching for a long time, but i can't find a solution to the problem. Any suggestions will be helpful!

Thank you, moshimojo